In a time of flight mass spectrometer (TOF-MS), for example, ions are accelerated and introduced into a flight space where no electric or magnetic field is present, and they are separated by their mass to charge ratios based on the time of flight until they enter an ion detector. For the ion source of the TOF-MSs, an ion trap is often used.
A typical ion trap 2 is composed of a ring electrode 21 and a pair of end cap electrodes 22 and 23, where the ring electrode 21 is placed between them, as shown in FIG. 4. Normally, a radio frequency (RF) voltage is applied to the ring electrode 21 to form a quadrupole electric field in an ion trap space 24 defined by the ring electrode 21 and the end cap electrodes 22, 23, whereby the quadrupole electric field traps and stores ions within the ion trap 2. Ions may be generated outside of the ion trap 2 and then introduced in it, or otherwise they may be generated within the ion trap 2. Theoretical explanation of an ion trap is given in, for example, R. E. March and R. J. Hughes, “Quadrupole Storage Mass Spectrometry”, John Wiley & Sons, 1989, pp. 31–110.
A wide variety of samples may be analyzed by a mass spectrometer, and the mass to charge ratio of ions to be analyzed by a mass spectrometer also varies largely. In the ion trap described above, not only the ions are stored in it, but also various other treatments are performed in it; e.g., the ion trapping potential is optimized, their vibration is cooled, ions of certain mass to charge ratio are selected, or selected ions are dissociated in order to analyze the structure of the ions.
When a mass analysis is to be done by the TOF-MS 3, the application of the RF voltage to the ring electrode 21 is stopped at the time when the object ions to be analyzed are prepared in the ion trap 2. Then a certain voltage is applied between the end cap electrodes 22 and 23 to form an ion ejecting electric field in the ion trap 2. Owing to the ion ejecting electric field, the ions are accelerated and ejected from the ion trap 2 through an ejection hole 26 of an end cap electrode 23. The ejected ions are analyzed by the TOF-MS 3.
In the mass analysis at the TOF-MS 3, the flight time until the ions are detected by the ion detector 31 of the TOF-MS 3 varies according to the starting point of the ions of the same mass to charge ratio. When the electric field for trapping ions is formed in the ion trap 2 as explained above, ions in it vibrate owing to the electric field. Since the vibration is caused by the interaction between the electric field and the electric charge of the ions, the kinetics of the ions is different in the same electric field depending on the polarity of the electric charge of the ions. Therefore the starting point of the ions when they are ejected from the ion trap 2 vary largely depending on the stopping time of the ion trapping RF voltage. This variation in the starting point causes a shift of the peaks of the mass spectrum, which makes the determination of the exact mass to charge ratio difficult and deteriorates the mass resolution of the mass spectrometer.